Network Working Group M. Nottingham
Internet-Draft Rackspace
Updates: 2616 (if approved) R. Fielding
Intended status: Standards Track Adobe
Expires: April 20, 2012 October 18, 2011
Additional HTTP Status Codesdraft-nottingham-http-new-status-02
Abstract
This document specifies additional HyperText Transfer Protocol (HTTP)
status codes for a variety of common situations.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 20, 2012.
Copyright Notice
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document authors. All rights reserved.
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Internet-Draft Additional HTTP Status Codes October 20111. Introduction
This document specifies additional HTTP [RFC2616] status codes for a
variety of common situations, to improve interoperability and avoid
confusion when other, less precise status codes are used.
Feedback should occur on the ietf-http-wg@w3.org mailing list,
although this draft is NOT a work item of the IETF HTTPbis Working
Group.
2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. 428 Precondition Required
This status code indicates that the origin server requires the
request to be conditional.
Its typical use is to avoid the "lost update" problem, where a client
GETs a resource's state, modifies it, and PUTs it back to the server,
when meanwhile a third party has modified the state on the server,
leading to a conflict. By requiring requests to be conditional, the
server can assure that clients are working with the correct copies.
Responses using this status code SHOULD explain how to resubmit the
request successfully. For example:
HTTP/1.1 428 Precondition Required
Content-Type: text/html
<html>
<head>
<title>Precondition Required</title>
</head>
<body>
<h1>Precondition Required</h1>
<p>This request is required to be conditional;
try using "If-Match".</p>
</body>
</html>
Responses with the 428 status code MUST NOT be stored by a cache.
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Internet-Draft Additional HTTP Status Codes October 20114. 429 Too Many Requests
This status code indicates that the user has sent too many requests
in a given amount of time ("rate limiting").
The response representations SHOULD include details explaining the
condition, and MAY include a Retry-After header indicating how long
to wait before making a new request.
For example:
HTTP/1.1 429 Too Many Requests
Content-Type: text/html
Retry-After: 3600
<html>
<head>
<title>Too Many Requests</title>
</head>
<body>
<h1>Too many Requests</h1>
<p>I only allow 50 requests per hour to this Web site per
logged in user. Try again soon.</p>
</body>
</html>
Note that this specification does not define how the origin server
identifies the user, nor how it counts requests. For example, an
origin server that is limiting request rates can do so based upon
counts of requests on a per-resource basis, across the entire server,
or even among a set of servers. Likewise, it might identify the user
by its authentication credentials, or a stateful cookie.
Responses with the 429 status code MUST NOT be stored by a cache.
5. 431 Request Header Fields Too Large
This status code indicates that the server is unwilling to process
the request because its header fields are too large. The request MAY
be resubmitted after reducing the size of the request header fields.
It can be used both when the set of request header fields in total
are too large, and when a single header field is at fault. In the
latter case, the response representation SHOULD specify which header
field was too large.
For example:
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Internet-Draft Additional HTTP Status Codes October 2011
HTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/html
<html>
<head>
<title>Request Header Fields Too Large</title>
</head>
<body>
<h1>Request Header Fields Too Large</h1>
<p>The "Example" header was too large.</p>
</body>
</html>
Responses with the 431 status code MUST NOT be stored by a cache.
6. 511 Network Authentication Required
This status code indicates that the client needs to authenticate to
gain network access.
The response representation SHOULD indicate how to do this; e.g.,
with an HTML form for submitting credentials.
The 511 status SHOULD NOT be generated by origin servers; it is
intended for use by intercepting proxies that are interposed as a
means of controlling access to the network.
Responses with the 511 status code MUST NOT be stored by a cache.
6.1. The 511 Status Code and Captive Portals
A network operator wishing to require some authentication, acceptance
of terms or other user interaction before granting access usually
does so by identify clients who have not done so ("unknown clients")
using their MAC addresses.
Unknown clients then have all traffic blocked, except for that on TCP
port 80, which is sent to a HTTP server (the "login server")
dedicated to "logging in" unknown clients, and of course traffic to
the login server itself.
For example, a user agent might connect to a network and make the
following HTTP request on TCP port 80:
GET /index.htm HTTP/1.1
Host: www.example.com
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Internet-Draft Additional HTTP Status Codes October 2011
Upon receiving such a request, the login server would generate a 511
response:
HTTP/1.1 511 Network Authentication Required
Refresh: 0; url=https://login.example.net/
Content-Type: text/html
<html>
<head>
<title>Network Authentication Required</title>
</head>
<body>
<p>You need to <a href="https://login.example.net/">
authenticate with the local network</a> in order to get
access.</p>
</body>
</html>
Here, the 511 status code assures that non-browser clients will not
interpret the response as being from the origin server, and the
Refresh header redirects the user agent to the login server (an HTML
META element can be used for this as well).
Note that the 511 response can itself contain the login interface,
but it may not be desirable to do so, because browsers would show the
login interface as being associated with the originally requested
URL, which may cause confusion.
7. Security Considerations7.1. 428 Precondition Required
The 428 status code is optional; clients cannot rely upon its use to
prevent "lost update" conflicts.
7.2. 429 Too Many Requests
Servers are not required to use the 429 status code; when limiting
resource usage, it may be more appropriate to just drop connections,
or take other steps.
7.3. 431 Request Header Fields Too Large
Servers are not required to use the 431 status code; when under
attack, it may be more appropriate to just drop connections, or take
other steps.
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Internet-Draft Additional HTTP Status Codes October 20119.2. Informative References
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
March 2007.
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
Appendix A. Acknowledgements
Thanks to Jan Algermissen for his suggestions and feedback.
The authors take all responsibility for errors and omissions.
Appendix B. Issues Raised by Captive Portals
Since clients cannot differentiate between a portal's response and
that of the HTTP server that they intended to communicate with, a
number of issues arise.
One example is the "favicon.ico"
<http://en.wikipedia.org/wiki/Favicon> commonly used by browsers to
identify the site being accessed. If the favicon for a given site is
fetched from a captive portal instead of the intended site (e.g.,
because the user is unauthenticated), it will often "stick" in the
browser's cache (most implementations cache favicons aggressively)
beyond the portal session, so that it seems as if the portal's
favicon has "taken over" the legitimate site.
Another browser-based issue comes about when P3P
<http://www.w3.org/TR/P3P/> is supported. Depending on how it is
implemented, it's possible a browser might interpret a portal's
response for the p3p.xml file as the server's, resulting in the
privacy policy (or lack thereof) advertised by the portal being
interpreted as applying to the intended site. Other Web-based
protocols such as WebFinger
<http://code.google.com/p/webfinger/wiki/WebFingerProtocol>, CORS
<http://www.w3.org/TR/cors/> and OAuth
<http://tools.ietf.org/html/draft-ietf-oauth-v2> may also be
vulnerable to such issues.
Although HTTP is most widely used with Web browsers, a growing number
of non-browsing applications use it as a substrate protocol. For
example, WebDAV [RFC4918] and CalDAV [RFC4791] both use HTTP as the
basis (for network filesystem access and calendaring, respectively).
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Internet-Draft Additional HTTP Status Codes October 2011
Using these applications from behind a captive portal can result in
spurious errors being presented to the user, and might result in
content corruption, in extreme cases.
Similarly, other non-browser applications using HTTP can be affected
as well; e.g., widgets <http://www.w3.org/TR/widgets/>, software
updates, and other specialised software such as Twitter clients and
the iTunes Music Store.
It should be noted that it's sometimes believed that using HTTP
redirection to direct traffic to the portal addresses these issues.
However, since many of these uses "follow" redirects, this is not a
good solution.
Authors' Addresses
Mark Nottingham
Rackspace
Email: mnot@mnot.net
URI: http://www.mnot.net/
Roy T. Fielding
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/
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